Optical and optoelectronic components produced with a silicon-based technology are key components, for example for optical telecommunications or for optical connections within or between microelectronics circuits. Examples of such components are couplers, waveguides, modulators and photodetectors, in particular.
Modern photodetectors use germanium (Ge) as a detector material due to its significantly better absorbance compared to silicon in the wavelength range used for optical communication (λ=1.3-1.6 μm). A PIN diode, so called, is often used as a detector. “P” stands for a p-doped region, “I” for an intrinsic region and “N” for an n-doped semiconductor region. Important parameters of the diode are dark current, photocurrent, responsivity and optical bandwidth.
Very fast germanium photodiodes with an intrinsic region width of a few hundred nanometers and with an optical bandwidth of 50 GHz and more have already been demonstrated.
Such germanium photodiodes are typically produced by epitaxial growth of a germanium layer directly on an SOI (silicon on insulator) substrate, which allows direct coupling of the detector to the waveguide made from the upper monocrystalline silicon layer of the SOI structure. The light to be detected can thus be supplied parallel to the waveguide-diode interface, which allows the diode to be independently optimized, to a degree, with regard to responsivity and optical bandwidth.
The diodes are mostly contacted by the contact metal directly touching highly doped germanium regions which laterally or vertically adjoin the intrinsic germanium region. This results in losses because of light absorption by the metal electrodes and due to light absorption by free charge carriers in the doped germanium regions, thus limiting the responsivity of the diodes.
In order to prevent such losses, a photodiode that avoids having highly doped germanium regions, and in which the diode is contacted with metal via doped non-absorbing silicon regions which touch the absorbing intrinsic germanium region from below, was presented in the publication by Yi Zhang, Shuyu Yang, Yisu Yang, Michael Gould, Noam Ophir, Andy Eu-Jin Lim, Guo-Qiang Lo, Peter Magill, Keren Bergman, Tom Baehr-Jones, and Michael Hochberg entitled “A high-responsivity photodetector absent metal-germanium direct contact”, May 5, 2014, Vol. 22, No. 9, DOI:10.1364/OE.22.011367, OPTICS EXPRESS 11367-11375, referred to hereinafter as [1]. One disadvantage of this prior art photodiode is that a high responsivity and a high bandwidth are not achieved until very high operating voltages greater than 4V are used.
This disadvantage was mitigated with a germanium PIN photodiode proposed in the publication by S. Sahni, N. K. Hon, and G. Masini entitled “The Dual-Heterojunction Ge on Si Photodetector”, ECS Transactions, Vol. 64, issue 6, pp. 783-789, referred to hereinafter as [2], but with acceptance of a relatively narrow bandwidth less than 50 GHz.
In summary, no germanium PIN photodiodes are known from the prior art that achieve bandwidths significantly greater than 50 GHz in combination with a responsivity of 1 A/W or higher (for λ˜1.55 μm) at an operating voltage which is acceptably low with regard to dark current and wiring, in particular an operating voltage of at most −2 V.